Peyman Sakhaii

Measurement of long range H,C couplings in natural products in orienting media: a tool for structure elucidation of natural products

In this paper we show that water insoluble compounds dissolved in poly-gamma-benzyl-glutamate are amenable to the measurement of a number of homo- and heteronuclear dipolar couplings. The sensitivity and experimental precision of dipolar couplings are sufficient to obtain a good match with the structure. In order to achieve the necessary precision for H,C dipolar couplings between protons and carbons that are not directly bound a new method for the measurement of heteronuclear long range couplings is introduced that allows a one-parameter fit to a HSQC-based experiment as reference experiment. The methodology is applied to menthol (1R, 3S, 4R).

Broadband homodecoupled NMR spectroscopy with enhanced sensitivity

A new type of broadband homodecoupling technique is described, which is based on the original version of the Zangger-Sterk experiment, but results in a spectrum with higher sensitivity. The homodecoupling is performed by a combination of selective and non-selective 180° RF pulses in the presence of weak rectangular pulsed field gradients in a pseudo 2D experiment. The proposed experiment uses a fast pulsing approach to increase the signal-to-noise ratio per unit time. The recycle delay is significantly shortened typically to about 100 ms. After each scan, the offset of the selective shaped pulse is changed to access fresh magnetisation from adjacent frequency/spatial regions. The physical acquisition time was limited to 40 ms to keep the total length of the pulse sequence as short as possible. Broadband inversion BIP pulses are used instead of 180° hard pulses. They are used pairwise to cancel out unwanted phase shifts over the bandwidth. Reconstruction of the homodecoupled spectrum was done by concatenating the first 10 ms of the FID from each single increment to obtain the final homodecoupled proton FID followed by Fourier transformation. The new method can either be used to acquire broadband homodecoupled spectra in a shorter time or to increase the signal-to-noise ratio compared to the original Zangger-Sterk experiment. Using eight different frequencies can thus lead to a signal to noise gain of a factor √8 or a factor of eight in time.

Faster and cleaner real-time pure shift NMR experiments.

Real-time pure shift experiments provide highly resolved proton NMR spectra which do not require any special processing. Although being more sensitive than their pseudo 2D counterparts, their signal intensities per unit time are still far below regular NMR spectra. In addition, scalar coupling evolution during the individual data chunks produces decoupling sidebands. Here we show that faster and cleaner real-time pure shift spectra can be obtained through the implementation of two parameter alterations. Variation of the FID chunk lengths between individual transients significantly suppresses decoupling sidebands for any kind of real-time pure shift spectra and thus allows for example the analysis of minor components in compound mixtures. Shifting the excitation frequency between individual scans of real-time slice-selective pure shift spectra increases their sensitivity obtainable in unit time by allowing faster repetitions of acquisitions.

Broadband homodecoupled heteronuclear multiple bond correlation spectroscopy.

A general concept for removing proton-proton scalar J couplings in 2D NMR spectroscopy is proposed. The idea is based on introducing an additional J resolved dimension into the pulse sequence of a conventional 2D experiment to design a pseudo 3D NMR experiment. The practical demonstration is exemplified on the widely used gradient coherence selected heteronuclear long-range correlation spectroscopy (HMBC). We refer to this type of pulse sequence as tilt HMBC experiment. For every (13)C chemical shift evolution increment, a homonuclear J resolved experiment is recorded. The long-range defocusing delay of the HMBC pulse sequence is exploited to implement this building block. The J resolved evolution period is incremented in a way very similar to ACCORDION spectroscopy to accommodate the buildup of heteronuclear long-range antiphase magnetisation as well. After Fourier transformation in all dimensions the spectra are tilted in the J resolved dimension. Finally, a projection along the J resolved dimension is calculated leading to almost disappearance of proton-proton spin multiplicities in the 2D tilt HMBC spectrum. The tilt HMBC experiment combines sensitivity with simple experimental setup and can be recorded with short recycle delays, when combined with Ernst angle excitation. The recorded spectra display singlet proton signals for long-range correlation peaks making an unambiguous signal assignment much easier. In addition to the new experiment a simple processing technique is applied to efficiently suppress the noise originating from forward linear prediction in the indirect evolution dimensions. In case of issues with fast repetition times, probe heating and RF power handling most of the RF pulses can be replaced by broadband, frequency swept pulses operating at much lower power.

Improving the sensitivity of conventional spin echo spectra by preservation of initial signal-to-noise ratio

A simple processing strategy is introduced to enhance the spectral quality and signal-to-noise ratio in conventional J resolved spectra. The idea of pseudo echo filtering is extended to conserve the primary signal-to-noise, predominating at the beginning of the FID in the indirect dimension. This is achieved by matching the maximum amplitude of the FID with that of the sine window function. Practically, the FID is right shifted by the number of acquired points in the indirect dimension; missing data points are backward predicted and finally multiplied with the unshifted sine window function. Standard processing tools are employed for this purpose. The results of data processing using different window functions with and without right shifts and back predictions are discussed. The signal-to-noise ratio of the J resolved spectrum is increased by a factor of 6 compared to standard data processing using pseudo echo filtering alone.

An alternative approach for recording of multidimensional NMR data based on frequency dependent folding mechanism

A new scheme for obtaining HSQC spectra with improved resolution or in a shorter time called SHARC (Shaped Arrayed data aCquisition protocol) is proposed, which uses region selective RF pulses and allows the sweep width to be adjusted individually for each region. It thus bypasses the problems with the Nyquist theorem associated with other method suggested for this purpose. Assignment of the cross-peaks to their respective region is achieved by manipulating the phases of the RF pulses and/or their frequencies. SHARC NMR can be applied without any previous knowledge of the chemical shift distribution, but can be further optimized on the basis of a quick overview spectrum.

Synthesis of a Novel Class of Carbohydrate-Containing Calix[4]resorcinarene Adopting an Asymmetrical Diamond Structure

A novel type of calixsugars, containing sugar moieties at the methine bridges of the calixarene is introduced. These calixsugars were prepared via a nonconvergent stepwise fragment condensation. Four new stereogenic centers were formed simultaneously, and only one diastereoisomer 5 was isolated. The condensation procedure is remarkably mild allowing for a large diversity of labile groups to be used. The solution structure of calixarene 5 with two D-glucose and two hexyl moieties was determined by NMR spectroscopy by means of NOEs and coupling constants for molecular dynamics (MD). Chemical shifts were used to validate the conformation with the least NOE and J violations. The structure of calixsugar 5 has the configuration rctc referring to one sugar residue (r=reference, c=cis, t=trans) and adopts a diamond conformation for the macrocyclic backbone with the two sugar moieties axial on opposite sides of the macrocyclic ring and the two hexyl groups on the same side.

A different approach to multiplicity-edited heteronuclear single quantum correlation spectroscopy.

A new experiment for recording multiplicity-edited HSQC spectra is presented. In standard multiplicity-edited HSQC experiments, the amplitude of CH2 signals is negative compared to those of CH and CH3 groups. We propose to reverse the sign of (13)C frequencies of CH2 groups in t1 as criteria for editing. Basically, a modified [BIRD](r,x) element (Bilinear Rotation Pulses and Delays) is inserted in a standard HSQC pulse sequence with States-TPPI frequency detection in t1 for this purpose. The modified BIRD element was designed in such a way as to pass or stop the evolution of the heteronuclear (1)JHC coupling. This is achieved by adding a 180° proton RF pulse in each of the 1/2J periods. Depending on their position the evolution is switched on or off. Usually, the BIRD- element is applied on real and imaginary increments of a HSQC experiment to achieve the editing between multiplicities. Here, we restrict the application of the modified BIRD element to either real or imaginary increments of the HSQC. With this new scheme for editing, changing the frequency and/or amplitude of the CH2 signals becomes available. Reversing the chemical shift axis for CH2 signals simplifies overcrowded frequency regions and thus avoids accidental signal cancellation in conventional edited HSQC experiments. The practical implementation is demonstrated on the protein Lysozyme. Advantages and limitations of the idea are discussed.

Small angle double quantum spectroscopy (SAQS NMR)

A new experiment for recording double quantum spectra is introduced. The 2D DQ NMR experiment yields phase sensitive spectra with double quantum frequencies in F1. The appearance of remote peaks is vastly suppressed by using a small flip angle double quantum excitation and reconversion. Pulse sequences and phase sensitive processing are discussed. The complexity of the SQ antiphase magnetization given in larger proton spin networks could be reduced by using the option of band selective decoupling during the preparation period. In addition, an ACCORDION element is applied by incrementing the J evolution delay in concert with the t1 period. With this the excitation of double quantum coherence over a wider range of J values is achieved. A broadband homodecoupled version of the DQ experiment is proposed, where correlation peaks with singlet response at F2 chemical shifts and double quantum frequencies in F1 are obtained. We call this experiment Small Angle double Quantum Spectroscopy SAQS NMR.